The invention relates to a method for transferring intersystem connections, in particular between two asynchronous radiocommunication systems. The invention is particularly suited to use in a mobile communication system or wireless subscriber line system.
In radiocommunication systems, for example in the second generation European mobile communication system GSM (Global System for Mobile Communications), information such as voice, image information or other data for example is transmitted via a radio interface using electromagnetic waves. The radio interface relates to a connection between a base station and a plurality of subscriber stations, wherein the subscriber stations may be mobile stations or fixed radio stations for example. The electromagnetic waves are emitted in this case using carrier frequencies lying in a frequency band provided for the respective system. For future radiocommunication systems, for example the UMTS (Universal Mobile Telecommunications System) or other third generation systems, frequencies are provided in the frequency band of approximately 2000 MHz. Two modes are provided for the third generation mobile communication system UMTS, with one mode delineating an FDD (frequency division duplex) mode and the other mode delineating a TDD (time division duplex) mode. Said modes are used in different frequency bands, with both modes supporting a so-called CDMA (Code Division Multiple Access) subscriber separation method.
For the observation of GSM radio cells based on the FDD mode of the third generation digital mobile communication system UMTS at the time of application, the following documents form the basis for international 3GPP standardization:
D1: TS 25.212 “Multiplexing and channel coding (FDD)”, V3.1.1, 1999-12, especially Chapter 4.4 “Compressed mode”,
D2: TS 25.215 “Physical layer—Measurements (FDD)”, V3.1.1, 1999-12, especially Chapter 6 “Measurements for UTRA FDD”, and
D3: RAN 25.231 “Physical layer—Measurements”, V0.3.0, 1999-06, especially Chapter 5.1.3 ff. “Measurements for the handover preparation from UTRA FDD at the UE”.
Descriptions of the second generation mobile communication system GSM are based on the book by J. Biala “Mobilfunk und Intelligente Netze”, Vieweg Verlag, 1995.
Owing to a co-existence and a desired harmonization between second and third generation radiocommunication systems, subscriber stations that have established a connection in one radiocommunication system should be provided with the capability to transfer the connection to a further radiocommunication system which in some cases may support a different transmission mode. An intersystem connection transfer of this type, also referred to as intersystem handover, requires that prior to the transfer the subscriber station must already be synchronized with the radiocommunication system that is to take over the connection. For this reason, signals of a so-called synchronization channel (SCH) are transmitted periodically by the base stations of the radiocommunication system in the radio coverage area, by which signals a subscriber station can synchronize with the time structure of the radio interface of the radiocommunication system and can subsequently carry out measurements, for example regarding the receiving level, which are taken into account for the transfer decision.
The FDD mode of the UMTS mobile communication system is based on a so-called W-CDMA subscriber separation method which is characterized by continuous transmitting and receiving on designated broadband transmission channels. In contrast to the known time slot structure of the GSM mobile communication system and of the TDD mode of the UMTS mobile communication system, no dedicated transmission pauses for measuring adjacent radio cells or parallel mobile communication systems operating in a different frequency band are available to a subscriber station in the FDD mode when changing over between transmitting and receiving.
One solution to this problem is the realization of a second receiving device in the subscriber station, but this disadvantageously results in increased costs, an additional space requirement as well as a higher power consumption of the subscriber station.
For this reason a concept was realized according to which a subscriber station having only one receiving device is also capable of detecting signals in other frequency bands, and of using them for example for an intrasystem or intersystem connection transfer. This concept is termed “compressed mode” and is explained inter alia in the referenced documents D1 to D3. With this mode, within a time frame of 10 ms, the information contained therein is compressed, inter alia by various methods such as puncturing and changing the spreading factor, in such a way that a transmission gap of a specific length is produced. Within said transmission gap, the subscriber station can tune the receiving device to another frequency band and receive and evaluate signals transmitted therein. The “compressed mode” can be performed both in the uplink direction and in the downlink direction.
However, this concept also has disadvantages because, for example, a higher transmitting power becomes necessary as a result of the reduction of the spreading factor in order to ensure a constant transmission quality. This increased transmitting power leads to increased interference disruption between connections concurrently established in the same frequency band.
Moreover, the concept disadvantageously breaks the closed loop for transmit power control. This runs counter to the principle of a DS-CDMA system (Direct Sequence CDMA), which requires a very fast and precise transmit power control for the uplink direction to ensure optimal capacity of the system by minimizing the respective transmitting power of the subscriber stations.
The number and periodicity of the time frames with transmission gaps are individually adjusted on the network side depending on the respective conditions and the current need for observing other frequency bands or systems.
As the future UMTS mobile communication system starts to become widespread, so-called multimode subscriber stations will support at least both the GSM standard and the FDD mode of the UMTS standard. This is important primarily for operators who implement, for example, both comprehensive coverage of an entire country with a GSM mobile communication system and an initially locally restricted coverage with the UMTS mobile communication system.
In comparison to the UMTS mobile communication system, the GSM mobile communication system has a significantly smaller frequency channel spacing—200 kHz in comparison to 5 MHz with FDD mode—as well as a greater frequency reuse factor—typically 7 as opposed to 1. This requires the observation of a greater number of adjacent radio cells, which must be observed in the case of an intersystem connection transfer from an FDD mode to a GSM system.
According to the GSM standard, for example the receiving level (RSSI—Received Signal Strength Indicator) of up to 32 adjacent cells must be observed by the subscriber station within a period of 30 seconds, and the six adjacent cells that offer the best transmission conditions must be signaled every 480 milliseconds to the currently supplying base station. In addition to this observation of the respective receiving levels, information of the respective control channel (BCCH—Broadcast Control Channel) must also be decoded and evaluated.
In the GSM mobile communication system, this problem is solved by averaging the measured RSSI within a respective time frame (4.6 ms) and by using a so-called idle frame, that is to say a time frame in which no transmission takes place, for detecting information of a selected radio cell.
In contrast, a subscriber station with a connection established in the FDD mode of the UMTS standard has no recourse to such concentrated measurements since a regular use of the compressed mode would lead to a significant reduction in the transmission quality. For this reason, it is anticipated that no generation of transmission gaps with a periodicity of 120 ms will be provided in the FDD mode.
The transmission gaps can however be used to observe a plurality of frequency bands in each case. In comparison with high-periodicity observation, this is more efficient since the required times for controlling the receiving device cause corresponding losses. Nevertheless, one entire transmission gap should be used exclusively for detecting the information of the control channel of an adjacent GSM radio cell.
Owing to the negative impact on the transmission quality set forth above, the compressed mode is not used permanently but rather, for example, the beginning and the extent of the measurements for a calculated need, to maintain an established connection for example, are determined and signaled to the subscriber station. For this decision regarding an activation or deactivation of the compressed mode, the use of a threshold value with which the respective current transmission quality of the connection is compared is proposed.
One disadvantage of this solution for controlling the connection transfer from the FDD mode to the GSM standard lies in its need for a large number of measurements with a concomitant decoding of information, resulting inter alia from the large frequency reuse factor and the large number of frequency channels.
With the aid of a diagram, FIG. 2 illustrates the effect of shifting the threshold value Th for the transmission quality. The transmission quality Q is plotted here against a signal-to-noise ratio (Eb/No), where Eb represents an averaged energy of an information bit of a transmission channel. The compressed mode is activated when the value falls below the respective threshold.
As can be seen from the diagram, with compressed mode activated it is only possible to achieve a poorer quality for the same Eb/No ratio, with a respective performance loss as a result.
If the threshold value is set low, as in example b of FIG. 2, then a large number of transmission gaps per time unit is required, which leads to a clear performance constraint as described. The large number of transmission gaps is necessary to perform all RSSI measurements and decoding in as short as possible a time before a possible loss of the connection. In the case where the subscriber station is near the radio cell boundaries and the transmission quality is generally already very low, this can lead to a premature loss of the connection.
If, on the other hand, the threshold value is set high, as in example a of FIG. 2, the additional degradation of quality results from compressed mode being activated at an earlier time, but in this case there is sufficient time available for recording and evaluating all the necessary measurements and information. In comparison with example b, this leads to more reliable information about adjacent radio cells potentially suitable for the connection transfer.
Current proposals proceed from a periodic insertion of transmission gaps into the continuous data transmission, with the transmission gaps being used in each case for RSSI determination and decoding of the information sequences of the control channels. However, if said transmission gaps are inserted only with a low periodicity, then the reliability of the information about potentially suitable adjacent radio cells is in turn reduced, and the probability increases of a decoding of radio cells having too low a transmission quality within the best six radio cells determined by the subscriber station.